Process for production of polyoxymethylene

ABSTRACT

A process for producing a polyoxymethylene using a reactor having a discharging port having a weir, which process comprises introducing trioxane or a mixture of trioxane and a compound reactive therewith into the reactor from its feeding port and continuously recovering the powdery polyoxymethylene formed, from said discharging port having a weir. Owing to the presence of the weir, the residence time of reaction mixture in reactor is long, making high the conversion of the monomer.

This application is a 371 of PCT/JP95/02161 filed Oct. 20, 1995.

TECHNICAL FIELD

The present invention relates to a reactor used for production of apolyoxymethylene widely used as an engineering plastic in electricalparts, automobile parts, mechanical parts, etc., as well as to a processfor producing a polyoxymethylene using the reactor. More particularly,the present invention relates to an improved process for producing apowdery polyoxymethylene continuously and efficiently using, as a mainstarting material, trioxane or a mixture of trioxane and a compoundreactive therewith.

BACKGROUND ART

Polyoxymethylenes have been produced by a method of subjecting trioxaneto bulk homopolymerization or subjecting trioxane with ethylene oxide ortrioxane with a cyclic formal such as dioxolane, trioxepane or the like,to bulk copolymerization in the presence of a cationically activecatalyst.

In conventional polyoxymethylene production, a reactor of two shaftsself-cleaning type, such as disclosed in JP-A-51-84890 has been usedmainly. In this reactor of two shafts self-cleaning type, thedischarging port for reaction mixture is provided at the bottom of thereactor and consequently the polyoxymethylene formed as an intendedproduct is recovered downward. This reactor, however, has had a problemin that the short residence time of reaction mixture gives a lowconversion. Hence, a number of proposals have been made in order to makethe residence time longer and the conversion higher.

For example, JP-A-56-59824 proposes a process wherein, in each twoadjacent paddles fixed to a rotary shaft of a reactor, the angle of one(back) paddle is advanced by a given amount from the angle of the other(precedent) paddle to control the transfer speed of reaction mixture toincrease the filling ratio of the reaction mixture and prolong theresidence time of the reaction mixture. Also, JP-A-61-238812 proposes aprocess wherein a reactor is lifted at the discharging port side so thatthe reactor is tilted as a whole by 1-10° from the horizontal state toprolong the residence time of the reaction mixture.

However, in the above process wherein the angle of one paddle isadvanced from the angle of the other paddle to prolong the residencetime, when the filling ratio is too high, the torque required forstirring increases sharply and surpasses the endurance limit of reactor(torque-out), making the continued reactor operation impossible. In theabove process wherein a reactor is tilted slightly as a whole, both avery high conversion and very stable operation can be achieved; however,L/D (L: length of reactor, D: inside diameter of reactor) must be largein order to make sufficiently long the residence time, which requires alarge reactor not suitable for industrial use.

The object of the present invention is to provide a process forproducing a polyoxymethylene, wherein the residence time of reactionmixture in a reactor is long and thereby the conversion of trioxane canbe increased.

DISCLOSURE OF THE INVENTION

The present inventors made a study in order to solve the above-mentionedproblems of the prior art and, as a result, found out that the aboveobject can be achieved by improving the structure of discharging port,discharging means for reaction mixture, of a reactor. The finding hasled to the completion of the present invention.

The present invention provides a process for producing apolyoxymethylene using a reactor comprising:

two parallel rotary shafts for stirring and reacting raw materials forreaction which shafts are each provided with a plurality of paddles androtated in the same direction,

a twin-cylindrical case having an inner wall along the circumferencesformed by the front ends of the individual paddles when the paddles arerotated, accommodating the two rotary shafts and provided with a heatingand cooling jacket,

a raw materials-feeding means for introducing trioxane or a mixture oftrioxane and a compound reactive therewith, which is provided at one endof the case, and

a discharging means for continuously recovering the powderypolyoxymethylene formed, which is provided at the other end of the case,said discharging means being a discharging port having a weir, whichprocess comprises introducing trioxane or a mixture of trioxane and acompound reactive therewith, from the raw materials-feeding means intothe case, stirring and reacting the raw materials for reaction to giverise to polymerization, and continuously recovering the powderypolyoxymethylene formed.

The reactor according to the present invention has a twin-cylindricalcase provided with a jacket capable of heating and cooling the case,which accommodates two rotary shafts along the two central axes of thecylinders. The two rotary shafts are each provided with a plurality ofpaddles and rotated in the same direction. The rotary shafts thereby canstir and react the raw materials in small gaps formed between each twofacing paddles fixed to the two different shafts and between each paddleand the inner wall of the case. The reactor further has, at one end, araw materials-feeding means for introducing a main raw material, i.e.trioxane or a mixture of trioxane and a compound reactive therewith intothe reactor and, at the other end, a discharging means for recoveringthe powdery polyoxymethylene formed.

The raw materials-feeding means is preferably a raw materials-feedingport.

In the present invention, the discharging means is preferably adischarging port having a weir. The discharging port is provided so asto extend nearly in a tangent direction to the circumference of thecylinder cross-section of the reactor, or in a radial direction of thecylinder cross-section, with the tangent direction being preferred. Thedischarging port is provided with a weir. The provision of the weirmakes longer the residence time of reaction mixture up to discharging,making it possible to increase the conversion of trioxane. Therefore,the weir in the present invention includes any measure capable ofcontrolling the time of reaction mixture up to discharging andprolonging the residence time of reaction mixture, whatever shape andsize the measure may have, and should not be limited to the embodimentsdisclosed in the examples described below and the accompanying drawings.

When the discharging port is provided in the tangent direction,preferably a downward tangent direction (hereinafter referred to aslateral direction), the height of the weir of the discharging port fromthe lowest position of the cylinder inner wall is 0.1 D to 0.8 D,preferably 0.3 D to 0.6 D wherein D is the inside diameter of thereactor cylinder. A weir height of less than 0.1 D has a small effect onthe prolongation of the residence time, making it impossible to increasethe conversion of trioxane. A weir height of more than 0.8 D results ininsufficient dischargeability for powdery polyoxymethylene andgeneration of torque-out, making the reactor operation impossible.

Meanwhile, when the discharging port is provided in the radialdirection, preferably a downward radial direction (hereinafter referredto as downward direction), the height H of the weir of the dischargingport from the lowest position of the cylinder inner wall is 0.1 D to 0.4D, preferably 0.2 D to 0.3 D. A weir height of less than 0.1 D has asmall effect on the prolongation of the residence time, making itimpossible to increase the conversion of trioxane. A weir height of morethan 0.4 D results in insufficient dischargeability for powderypolyoxymethylene and generation of torque-out, making the reactoroperation impossible. A weir height of 0.5 D invites contact between theweir and the rotary shaft; thus, a weir height of 0.5 D or more isimpossible to adopt actually. Since the discharging port is provided inthe downward direction from the cylindrical case of the reactor, part ofthe paddle(s) in the vicinity of the discharging port, may have to beremoved to fit a weir to the inner wall of the cylindrical case. In thatcase, a spacer ring 8 or the like can be used in place of the removedpaddle for fixation of the paddles present after the discharging port.

In the reactor of the present invention, the residence time can becontrolled by the height of the weir, irrelevantly to the feeding forceof paddles. Even when there is sudden increase in amount of rawmaterials fed, the flow of the reaction mixture in reactor is dammed upby the weir, and the reaction mixture can be smoothly discharged fromthe discharging port since the upper surface of the reaction mixture israised as a whole. Thus, the present reactor can solve the problemsassociated with the conventional reactors. In the present reactor, theweir can be a jacket type so as to enable heating or cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

The reactor according to the present invention is hereinafter describedin detail with reference to the accompanying drawings.

FIG. 1 is a partly sectional view of an example of the reactor accordingto the present invention.

FIG. 2 is a cross-sectional view of the reactor of FIG. 1, taken at theB--B line.

FIG. 3 is a view when the cross-section at the A--A line of FIG. 2 isseen from the arrow direction, where the section of a weir 7 formed inthe discharging port provided in a lateral direction to the reactorcylinder is seen from the side of the reactor. In FIG. 3, the height Hof the weir 7 is 0.5 D wherein D is the inside diameter of the reactorcylinder.

FIG. 4 is a view when the section of a weir 7 formed in the dischargingport provided in the downward direction from the reactor cylinder isseen from the side of the reactor. In FIG. 4, the height of the weir is0.25 D.

BEST MODE FOR CARRYING OUT THE INVENTION

In the reactor according to the present invention, two parallel rotaryshafts 2,2' are accommodated in a twin-cylindrical case 1 having ahollow inside having a 8-shaped section formed by the overlapping of twoeccentric circles of the same diameter. Outside the twin-cylinder isprovided a single jacket 4 or a jacket 4 split into a plurality ofportions, both for temperature control. To each of the two rotary shafts2,2' are fixed a plurality of paddles 3,3' each having a convexlens-shaped section. All the paddles are rotated at the same speed inthe same direction so that the front ends of each paddle can scratch theinner wall of the cylindrical case and the surface of a paddle facingthe former paddle, fixed to the other shaft. This reactor is acontinuous stirrer and mixer of self-cleaning type.

The sectional shape of each paddle may be, besides the above-mentionedconvex lens shape, a pseudopolygonal shape such as pseudotriangularshape or the like. The thickness of each paddle is preferably about 1/30to 1/2 of the inside diameter of the reactor cylinder. Each paddle isfixed to either of the two rotary shafts.

The gap between the front ends of each paddle and the inner wall of thecylindrical case is 2% or less, preferably 1% or less of the paddlemajor axis. The gap between one front end of any paddle fixed to onerotary shaft and the side of the paddle facing said paddle, fixed to theother rotary shaft is five times or less, preferably two times or lesssaid gap between the front ends of each paddle and the inner wall of thecylindrical case.

As shown in FIG. 2, the discharging port 6 is provided in a lateraldirection to the reactor cylinder and is fitted with a weir 7 so thatthe residence time of reaction mixture can be controlled by changing theheight H of the weir 7. The height H of the weir 7 is 0.1 D to 0.8 D,preferably 0.3 D to 0.6 D wherein D is the inside diameter of thereactor cylinder.

In carrying out the present invention, first the rotary shafts 2,2' ofreactor shown in FIG. 2 are rotated at a desired number of revolution.The number of revolution is not particularly restricted but is setgenerally at 10-150 rpm.

Through the jacket 4 of reactor shown in FIG. 1 is allowed to flow acooling water of 5-95° C. to remove the heat generated bypolymerization. When the jacket is split into a plurality of portions,cooling waters of different temperatures may be allowed to flow throughsaid portions, and the desired cooling water temperatures are 65-95° C.in the former half jackets and 5-40° C. in the latter half jackets.

Into the reactor of FIG. 1 in which the two rotary shafts are beingrotated and a cooling water(s) of desired temperature(s) is (are) beingallowed to flow through the jacket, are fed raw materials such astrioxane and the like from the raw materials-feeding port 5 of thereactor.

Into the feeding port 5 are fed liquid trioxane or a mixture of liquidtrioxane and a comonomer (e.g. ethylene oxide or 1,3-dioxolane), apolymerization catalyst and a molecular weight modifier (e.g. methylal).When the comonomer is ethylene oxide, it is fed preferably by dissolvingit in trioxane because it is a gas at normal temperature.

There takes place bulk homopolymerization of trioxane or bulkcopolymerization of trioxane and a comonomer. In this reaction, thereaction mixture changes from a high-fluidity liquid to a viscous liquidat the early stage and, when the polymerization proceeds and theconversion of trioxane reaches about 40% or more, a solid bulkpolyoxymethylene is formed. In this reactor, the reaction mixture is ahigh-fluidity or viscous liquid at the reactor portion close to thefeeding port; when the reaction proceeds, a bulk polyoxymethylene isformed and, simultaneously therewith, is ground by the paddles andbecomes a powdery polyoxymethylene; the powdery polyoxymethylene ismoved towards the discharging port 6 of reactor while the polymerizationproceeds further, and is taken out from the discharging port 6.

The process of the present invention is used for trioxanehomopolymerization or copolymerization. When the present process isintended for copolymerization, it is preferable to use, as a main rawmaterial, such a trioxane/comonomer mixture as to give a copolymerhaving a polyoxymethylene main chain containing 0.1-20 mole % ofoxyalkylene units having 2 or more carbon atoms. The comonomer includescyclic ethers and cyclic formals such as ethylene oxide, 1,3-dioxolane,1,4-butanediol formal, trioxepane and the like.

As the polymerization catalyst, cationic polymerization catalysts ofordinary use are used. Preferable of such catalysts are borontrifluoride, a boron trifluoride hydrate and a boron trifluoride-ethercomplex, and more preferable are boron trifluoride-diethyl ether complexand boron trifluoride-dibutyl ether complex.

The polymerization temperature is in a range of 60-130° C., preferably65-115° C.

With the reactor according to the present process, a powderypolyoxymethylene can be obtained efficiently and continuously andlong-term continuous operation with no trouble is possible.

The present invention is hereinafter described in more detail withreference to Examples.

Example 1 (present invention)

A reactor shown in FIG. 1 was used. The reactor has a cylinder insidediameter of 128 mm and a L/D (length/diameter) ratio of 8, and tworotary shafts are each provided with a feed screw having a length of 1 Dright under the raw materials-feeding port 5.

The reactor has paddles 3,3' as shown in FIG. 2. The gap between thefront ends of each paddle and the inner wall of the cylindrical case is4 mm or less. The paddles are fitted after the feed screw with the angleof any paddle being advanced by 45° in the rotational direction from theangle of the precedent paddle. The height of the weir of the dischargingport is 0.5 D.

Into the reactor were fed 20 kg/hr of liquid trioxane, 400 g/hr of1,3-dioxolane and a cyclohexane solution containing 0.02 mM, per mole oftrioxane, of boron trifluoride-dibutyl ether complex and 1 mM, per moleof trioxane, of methylal. The polymerization temperature was 90° C., andthe rotational speed of each front end of paddle was 3 m/min. From thedischarging port was obtained a fine-particle polymer containing 15% ofunreacted materials, which passed through a 10-mesh sieve.

No abnormality was encountered even after the 300-hour continuousoperation.

Example 2 (present invention)

Using the same reactor as in Example 1, an operation was started underthe same conditions with respect to amounts of raw materials fed,polymerization temperature, etc. as in Example 1 except that the heightH of the weir 7 of the discharging port 6 was changed to 0.3 D.

From the discharging port was obtained a fine-particle polymercontaining 18% of unreacted materials, which passed through a 10-meshsieve.

No abnormality was encountered even after the 300-hour continuousoperation.

Example 3 (present invention)

Using the same reactor as in Example 1, an operation was started underthe same conditions with respect to amounts of raw materials fed,polymerization temperature, etc. as in Example 1 except that the heightH of the weir 7 of the discharging port 6 was changed to 0.7 D.

From the discharging port was obtained a fine-particle polymercontaining 13% of unreacted materials, which passed through a 10-meshsieve.

Example 4 (present invention)

Using the same reactor as in Example 1, an operation was started underthe same conditions with respect to amounts of raw materials fed,polymerization temperature, etc. as in Example 1 except that the1,3-dioxolane used in Example 1 was replaced by 1,4-butanediol formal.

From the discharging port was obtained a fine-particle polymercontaining 14% of unreacted materials, which passed through a 10-meshsieve.

No abnormality was encountered even after the 300-hour continuousoperation.

Example 5 (present invention)

An operation was started under the same conditions with respect toamounts of raw materials fed, polymerization temperature, etc. as inExample 1, using the same reactor as in Example 1 except that thedirection of the discharging port 6 was made in a downward direction tothe cylindrical case of the reactor and the height H of the weir 7 wasmade 0.25 D as shown in FIG. 4.

From the discharging port was obtained a fine-particle polymercontaining 19% of unreacted materials, which passed through a 10-meshsieve.

No abnormality was encountered even after the 300-hour continuousoperation.

Example 6 (comparison)

An operation was started under the same conditions with respect toamounts of raw materials fed, polymerization temperature, etc. as inExample 1, using the same reactor as in Example 1 except that thedischarging port was provided with no weir.

From the discharging port was obtained a fine-particle polymercontaining 29% of unreacted materials, which passed through a 10-meshsieve.

Example 7 (comparison)

An operation was started under the same conditions with respect toamounts of raw materials fed, polymerization temperature, etc. as inExample 1, using the same reactor as in Example 1 except that thereactor was tilted by 50 so that the discharging port side of thereactor was higher.

From the discharging port was obtained, one hour after the start of theoperation, a fine-particle polymer containing 28% of unreactedmaterials, which passed through a 10-mesh sieve.

What is claimed is:
 1. A process for producing polyoxymethylene using areactor comprising:two parallel rotary shafts for stirring and reactingraw materials for the reaction which shafts are each provided with aplurality of paddles and rotated in the same direction, atwin-cylindrical case having an inner wall along the circumferencesformed by the front ends of the individual paddles when the paddles arerotated, accommodating the two rotary shafts and provided with a heatingand cooling jacket, a raw materials-feeding means for introducingtrioxane or a mixture of trioxane and a compound reactive therewith,which is provided at one end of said case, and a discharging means forcontinuously recovering the powdery polyoxymethylene formed, which isprovided at the other end of said case, said discharging means being adischarge port having a weir, wherein the discharging port of thereactor extends in a tangential direction to a circumference of one ofsaid twin-cylindrical cases, and a height of said weir is between 0.5and 0.8 D, wherein D is the inside diameter of said one cylindricalcase, which process comprises introducing trioxane or a mixture oftrioxane and a compound reactive therewith, from the rawmaterials-feeding means into the case, stirring and reacting the rawmaterials for reaction give rise to polymerization, and continuouslyrecovering the powdery polyoxymethylene formed, by passing the powderypolyoxymethylene over the weir at a height that is at least about 0.5 D.